1. Definition
The Chief Ray Angle (CRA) refers to the angle between the chief ray and the normal to the image plane.

• Chief Ray: The ray originating from an object point and directed towards the center of the aperture stop is called the chief ray. Corresponding to the chief ray, the ray passing through the edge of the aperture stop is called the marginal ray.
  
• Optical Axis: The geometric axis of symmetry of the optical system. For on-axis field points, the chief ray usually coincides with the optical axis; for off-axis field points, the chief ray deviates from the optical axis.
  
• Note the distinction: The incident angle is used to describe the angle between any arbitrary ray and the normal to the corresponding surface; whereas the Chief Ray Angle (CRA) specifically refers to the incident angle of the chief ray at the image plane interface.

2. CRA Matching
Lens CRA:
The exit angle of the chief ray determined by the lens structure, which forms a continuously increasing curve as the image height increases; the single value specified in the datasheet generally refers to the maximum exit angle at the maximum image height on the sensor.

Sensor CRA:
The optimal incident angle reference curve specified in the image sensor datasheet, which varies continuously with image height. It is used to determine the microlens shift, ensuring that incident light at the corresponding tilt angle falls precisely on the center of the photodiode.

Matching Relationship and Imaging Impact
(1) Ideal Matching (Lens CRA ≈ Sensor CRA)
The tilt angle of the light projected by the lens onto the sensor surface perfectly matches the preset deflection angle of the microlenses. After refraction, the light is precisely focused on the center of the photodiode (PD), optimizing the quantum efficiency (QE) of each pixel. The image brightness and color remain uniform across the entire field of view, achieving the optimal signal-to-noise ratio.

(2) Lens CRA < Sensor CRA (Lens exit light tilt angle is too small)
Because the preset shift of the microlenses fails to match the actual light rays, some incident rays fall outside the active area of the photodiode after refraction. This leads to insufficient light reception at the edges, causing vignetting, where the four corners of the image appear dark (as shown in the figure below).

(3) Lens CRA > Sensor CRA (Lens exit light tilt angle is too large)
The degree of light tilt exceeds the compensation capability of the microlenses. The light beam is blocked by the pixel metal wiring or undergoes optical crosstalk. This not only reduces light collection efficiency but also causes color crosstalk (as shown in the figure below) and color shading. This defect is difficult to fully correct through backend ISP algorithms.

3. CRA and Microlens Shift
This section focuses on front-side illuminated (FSI) CMOS sensors.
Modern CMOS image sensors integrate a microlens array above the pixels to converge incident photons and improve the light utilization efficiency of the photodiode (PD). The Chief Ray Angle (CRA) of incident light is larger at the edge positions of the sensor. If the microlenses of all pixels are centered and aligned with the photodiodes, light rays with large tilt angles will be blocked by the internal metal wiring of the pixels and fail to reach the photosensitive area, causing severe edge luminance vignetting. To solve this problem, the microlenses of the edge pixels can be physically shifted towards the incident direction of the chief ray. The shift amount increases synchronously with the CRA, guiding the tilted light to converge on the photodiode, thereby improving the pixel light collection efficiency and optimizing the luminance uniformity of the image.

See Also
入瞳, 均匀性_shading